The present invention relates to an apparatus for frequency deviation estimation and used in a receiver in the field of radio communication such as mobile communication, satellite communication. More particularly, this invention relates to an apparatus for frequency deviation estimation and estimates frequency deviation that occurs due to the instability in frequency of a local oscillator as well as to a method of estimating such frequency deviation.
A conventional frequency deviation estimation apparatus will now be explained. For example, among receivers that are used in mobile communication, there is one in which there is used a coherent detection system, wherein excellent detection characteristics are obtained even from a channel having a low carrier to noise ratio (C/N). In this coherent detection system, the carrier whose frequency is synchronized with the frequency of the carrier of the received signal are carrier estimated, whereby a detection output is obtained according to these estimated carriers. However, in the receiver adopting such a coherent detection system, oscillation frequency fluctuates due to the precision of the oscillator or the fluctuation in the temperature, etc. As a result, a deviation in the frequency is generated between transmitted signal and the received one. The phase of the signal point in the IQ plane (the complex plane defined by a real axis and an imaginary axis) gets rotated when such a frequency deviation exists.
In order to reduce such frequency deviation and thereby realize the enhancement of the synchronization characteristic, a frequency deviation estimation apparatus which can measure the rotation of phase from the received signal and estimate the frequency deviation from this measurement result is required to be used in the receiver. The received signal that has been modulated by the transmitter also contains the rotation of phase resulting from the modulation of the information along with the rotation of phase resulting from the existence of the frequency deviation. Thus, it becomes necessary to eliminate the component corresponding to the modulation of the information in order to estimate the frequency deviation.
For example, as a method of estimating the frequency deviation, there is one that uses a known sequence part contained in the received signal. This method is described in Japanese Patent Application Laid-Open Publication No. HEI 5-68062 entitled xe2x80x9cFrequency Offset Compensation Methodxe2x80x9d. FIG. 10 illustrates the construction of a conventional frequency deviation estimation apparatus (corresponding to the coherent detection circuit of Japanese Patent Application Laid-Open Publication No. HEI 5-68062). In FIG. 10, the reference numeral 101 denotes a received signal containing therein a known sequence. The reference numeral 102 denotes an average-phase calculating circuit that calculates an average phase of a partial known sequence which is a part of the known sequence. The reference numeral 103 denotes an average-phase differential circuit for obtaining a difference between n-th average phase and (n+1)-th average phase. The reference numeral 104 denotes an averaging circuit for obtaining an average of the differences so as to calculate the amount of deviation in the phase which is proportional to the frequency deviation.
The operation of the above-described conventional frequency deviation estimation apparatus will be explained hereafter. A known sequence that is previously known on the receiver side is periodically present in the received signal 101. Using this partial known sequence containing a part of a known sequence, the average phase in this portion corresponding to the partial known sequence is determined in the average-phase calculating circuit 102. This determination is made using an RLS algorithm, i.e. recursive least squares method. Further, in the average-phase calculating circuit 102, using other different partial known sequence pieces, a plurality of average phases xcex8k (k=1, 2, . . . , n, n+1) in the different averaging sections are obtained. It becomes possible to eliminate the information modulation in units of a symbol from the transmitted symbol sequence that is the original sequence of which information modulation is made, because it is known as a known sequence beforehand on the receiving side. Therefore, average phases can be easily determined.
In the average-phase differential circuit 103 corresponding to each average-phase calculating circuit 102 a difference between the k-th average phase xcex8k and the (k+1)-th average phase xcex8k+1 is calculated to obtain the received phase difference portions xcex94xcex8k (k=1, 2, . . . , n, n+1). Finally, the averaging circuit 104 averages the plurality of phase difference portions xcex94xcex8k to obtain the amount of deviation in the phase which is proportional to the frequency deviation.
However, in the above-described conventional frequency deviation estimation apparatus, intersymbol interference (the interference between adjacent symbols) sometimes occurs according to the circumstances of the channel. When applying FIG. 10 to the received signal that has been received via the channel in which intersymbol interference exists, there is the problem that it becomes impossible to correctly eliminate the modulation of the information in the average-phase calculating circuit 102. Since correct average phases cannot be obtained, the frequency deviation estimation characteristic of the frequency deviation estimation apparatus get deteriorated.
Further, in the RLS algorithm executed in the average-phase calculating circuit 102 of the conventional frequency deviation estimation apparatus tap gain is so controlled as to minimize the accumulation of the squared values of the frequency deviation that are made until tap gains are respectively updated. As a result, an optimum tap gain is reached with a high speed. However, the recursive least squares method adopted herein requires a large amount of calculation, raising the problem that the apparatus was very inefficient from the viewpoint of the construction of the hardware and the consumption of the power.
The present invention has been made in view of the above and has an object to provide an apparatus for frequency deviation estimation, which can realize the enhancement of the frequency deviation estimation characteristic of the received signal, the received signal being one wherein intersymbol interference has occurred, which can realize the simplification of the hardware and the reduction in the power consumption by determining the CIR (Channel Impulse Response) with a lesser amount of calculation, and which can realize optimum estimation of the frequency deviation by adjusting the detection precision and detection range regarding the frequency deviation, and a method of realizing the same.
According to a first aspect of the frequency deviation estimation apparatus that receives a signal containing therein a known sequence having an elementary pattern repeated therein, a channel impulse response estimating unit estimates channel impulse responses at a plurality of timings according to the elementary pattern in the known sequence contained in the received signal, and a frequency deviation calculating unit calculates frequency deviation according to the estimated channel impulse responses. Therefore, the modulation of the information can highly precisely be eliminated from the signal received via the channel in which intersymbol interference exists, and correct channel impulse response (that can be determined using a known method such as an RLS algorithm) can be obtained accordingly.
According to this invention, a channel impulse response estimating unit having a correlation unit that determines the channel impulse response (CIR) with a lesser amount of calculation is provided so that it is possible to achieve the simplification of the hardware and simultaneously realize the large reduction in the power consumption. Further, a frequency deviation calculating unit is provided so that a trade-off between the frequency deviation detection precision and the frequency deviation detection range becomes possible. Therefore, it is possible to optimally estimate the frequency deviation accordingly.
According to a second aspect of the frequency deviation estimation apparatus that receives a signal containing therein a known sequence having an elementary pattern repeated therein, a first channel impulse response estimating unit estimates a first channel impulse response according to the elementary pattern in the known sequence contained in the received signal, a second channel impulse response estimating unit estimates a second channel impulse response according to a sequence prepared by performing cyclic-shifting of the known sequence, and a frequency deviation calculating unit calculates frequency deviation according to the first and second channel impulse responses. Therefore, the modulation of the information can highly precisely be eliminated from the signal received via the channel in which intersymbol interference exists, and more correct channel impulse response (that can be determined using a known method such as an RLS algorithm) can be obtained accordingly.
According to this invention, a first channel impulse response estimating unit having a first correlation unit that determines the channel impulse response (CIR) with a lesser amount of calculation and a second channel impulse response estimating unit having a second correlation unit are provided so that it is possible to achieve the simplification of the hardware and simultaneously realize the large reduction in the power consumption. Further, a desired detection precision and detection range of frequency deviation can be set more in detail, with the result that it is possible to optimally estimate the frequency deviation accordingly.
According to this invention, a complex-adder performs complex addition of the complex conjugate products that have been calculated in the phase difference vector calculating unit. This equivalently means maximum ratio combining the rotating amounts of phase that have been procured from the channel impulse response. Therefore, the averaging effect for suppressing the noises is obtained. Also, in the deviation calculating unit, the frequency deviation detection precision is proportional to the timing-to-timing interval at which the CIR is procured and, further, the frequency deviation detection range is inversely proportional to the timing-to-timing interval at which the CIR is procured. Namely, these timing-to-timing intervals function as the trade-off parameters between the frequency deviation detection precision and the frequency deviation detection range. As a result of this, it becomes possible to adjust the frequency deviation detection precision and the frequency deviation detection range in correspondence with the status of use, which makes it possible to realize optimum estimation of the frequency deviation.
According to this invention, in one frequency deviation estimating unit, since the detection precision and the detection range are in the relationship of trade-off, either one of them is preferentially selectively determined. On this account, the invention is arranged to be equipped with a plurality of frequency deviation estimating unit having set therein respectively different estimation precision degrees and estimation ranges. In addition, the frequency deviation that are output from the respective frequency deviation estimating unit are combined by the frequency deviation combining unit. As a result of this, it is possible to realize a higher-precision and wider-range-of-frequency-band frequency deviation estimation apparatus.
According to this invention, a channel impulse response estimating unit having a correlation unit that determines the channel impulse response (CIR) with a lesser amount of calculation is provided so that it is possible to achieve the simplification of the hardware and simultaneously realize the large reduction in the power consumption. Further, a frequency deviation calculating unit is provided so that it becomes possible to perform the trade-off between the frequency deviation detection precision and the frequency deviation detection range and to optimally estimate the frequency deviation accordingly.
According to this invention, the modulation of the information can be highly precisely eliminated from the signal received via the channel in which intersymbol interference exists, and correct channel impulse response (that can be determined using a known method such as an RLS algorithm) can be obtained accordingly. Therefore, it is possible to greatly enhance the frequency deviation estimation characteristic.
According to this invention, a channel impulse response estimating step is provided that determines the channel impulse response (CIR) with a lesser amount of calculation so that it is possible to achieve the simplification of the hardware and simultaneously realize the large reduction in the power consumption. Further, a frequency deviation calculation step is provided so that it becomes possible to perform the trade-off between the frequency deviation detection precision and the frequency deviation detection range and to optimally estimate the frequency deviation accordingly.
According to this invention, the modulation of the information can be highly precisely eliminated from the signal received via the channel in which intersymbol interference exists, and more correct channel impulse response (that can be determined using a known method such as an RLS algorithm) can be obtained accordingly. Therefore, it is possible to greatly enhance the frequency deviation estimation characteristic.
According to this invention, a first channel impulse response estimating step and the second channel impulse response estimating step are provided that determines the channel impulse response (CIR) with a lesser amount of calculation so that it is possible to achieve the simplification of the hardware and simultaneously to realize the large reduction in the power consumption. Further, a desired detection precision and detection range of frequency deviation can be set more in detail, with the result that it is possible to optimally estimate the frequency deviation accordingly.
According to this invention, a complex-adding step is provided which performs complex addition of the complex conjugate products that have been calculated in the phase difference vector calculating step. This becomes equivalent to maximum ratio combining the rotating amounts of phase that have been procured from the channel impulse response. As a result, it is possible to obtain the averaged effect for suppressing the noises. Also, in the deviation calculating step, the frequency deviation detection precision is proportionate to the timing-to-timing interval at which the CIR is procured and, further, the frequency deviation detection range is inversely proportionate to the timing-to-timing interval at which the CIR is procured. Namely, here, these timing-to-timing intervals function as the trade-off parameters between the frequency deviation detection precision and the frequency deviation detection range. As a result of this, it becomes possible to adjust the frequency deviation detection precision and the frequency deviation detection range in correspondence with the status of use, which makes it possible to realize optimum estimation of the frequency deviation.
According to this invention, in one frequency deviation estimating step, since the detection precision and the detection range are in the relationship of trade-off, either one of them is preferentially selectively determined. On this account, a plurality of frequency deviation estimating steps are provided so as to be able to set therein respectively different estimation precision degrees and estimation ranges. In addition, the frequency deviation that are output in the respective frequency deviation estimating steps are combined in the frequency deviation combining step. As a result of this, it is possible to realize a higher-precision and wider-range-of-frequency-band frequency deviation estimating method.
According to this invention, a channel impulse response estimating step is provided that determines the channel impulse response (CIR) with a lesser amount of calculation so that it is possible to achieve the simplification of the hardware and simultaneously realize the large reduction in the power consumption. Further, a frequency deviation calculating step is provided so that it becomes possible to perform the trade-off between the frequency deviation detection precision and the frequency deviation detection range and to optimally estimate the frequency deviation accordingly.
According to this invention, in a complex-adding step the complex addition of the complex conjugate products that have been calculated in the phase difference vector calculating step is performed. This becomes equivalent to maximum ratio combining the rotating amounts of phase that have been procured from the channel impulse response. As a result, it is possible to obtain the averaging effect for suppressing the noises. Also, in the deviation calculating step, the frequency deviation detection precision is proportionate to the product of the timing-to-timing intervals at which the CIRs are procured and, further, the frequency deviation detection range is inversely proportionate to the timing-to-timing interval at which the CIR is procured. Namely, here, these timing-to-timing intervals function as the trade-off parameters between the frequency deviation detection precision and the frequency deviation detection range. As a result of this, it becomes possible to adjust the frequency deviation detection precision and the frequency deviation detection range in correspondence with the status of use, which makes it possible to realize optimum estimation of the frequency deviation.